The occurrence of the magnitude 7.9M Wenchuan earthquake on 12 May 2008, at the eastern foothill of the Tibetan Plateau, surprised many in the earthquake community. More intriguingly, the large number of aftershocks related to that earthquake almost exclusively occurred northeast of the main shock along the Longmenshan Fault Zone (LFZ), a phenomena that current models have trouble explaining.
In a recently published article we combine gravity modelling with a synthesis of an array of geological and geophysical observations to establish a regional hypothesis featuring (1) contrasting behaviors between the southern and central–northern LFZ, with the southern LFZ being a crustal–scale thrust zone, whereas the central–northern LFZ being a lithospheric–scale fault zone with strong episodic dextral transpressional motions; (2) such contrasting behaviors are driven by differential motions of two crustal blocks from the western highland (the Tibetan Plateau), pushed by India’s northward motion; (3) as a consequence, the LFZ features a twisted fault plane with the Wenchuan earthquake being located at the junction of the LFZ and the bounding fault between the two crustal blocks to the west; and (4) the LFZ has been behaving like this since around 40 Myr ago and it became an external boundary for a northeasterly-directed extrusion-style growth of the Tibetan Plateau.
A model showing an extrusion-styled growth of the Tibetan Plateau toward the northeast, bounded by the left-lateral ATF to the northwest and the dominantly right-lateral central and northern LFZ to the southeast. We suggest that this mode of plateau growth started at ca. 40 Ma. Bold red arrows symbolize directions of the extrusion-style growth of the Tibetan Plateau. Thin blue arrows show generalized GPS measurements of present-day crustal motions [Zhang et al., 2004]. Thin violet arrows represent observed cumulative postseismic surface displacements during the first 7 years after the Wenchuan earthquake [Diao et al., 2018]. Ages indicate the estimated timing of dominant plateau uplift for a given region, bounded by red dashed lines [Richardson et al., 2008]. Rates of relative motion along the margins of the Tibetan Plateau are from Bendick et al.  and Li et al. . Purple numbers on contour lines are depths in kilometers of the Cenozoic foreland basin deposits in both the southwestern Sichuan Basin and southern Tarim Basin [Wang et al., 2006]. MIN, Min Mountains; Qilian, Qilian Mountains; DA, Danba anticline.
Figure showing contrasting structure, topography and behaviors between the southern and the central-northern LFZ. (a) Subdivision of crustal blocks in the eastern Tibetan Plateau (after Burchfiel et al. ) and positions of the two lithospheric block diagrams as in (b). Focal solutions of the M7.9 Wenchuan earthquake (W) and aftershocks (orange dots), and that of the the M6.7 Ya’an event (Y), are shown. D – Danba Block; SP – Songpan Block; XS – Xue Shan Block (after Burchfiel et al. , Xu et al.  and Guo et al. ). (b) A 3D tectonic model for the eastern margin of Tibet along the LFZ. The 895 earthquake events of magnitude > 3.5 ruptured within ~100 km of transects L and LL (positions shown in (a)) between August 1933 and April 2015 (http://earthquake.usgs.gov/earthquakes/search/) are projected onto the two transects. The purple dot line marks the boundary between the Danba and Songpan crustal blocks – note the contrasting topography on the two sides of this boundary fault. Major faults are constrained by earthquake distribution and published seismic profiles [Robert et al., 2010a, 2010b; Guo et al., 2013]. (c) A schematic diagram illustrating a twisted fault plane along the LFZ (after Shen et al. ) driven by different block motion directions along the eastern margin of the Tibetan Plateau, with the hypocenter of the M7.9 Wenchuan earthquake (W) located near the twisting zone. The thrust plane in southern LFZ is within the crust, whereas in central and northern LFZ the increasingly more strike-slip fault cuts through the lithosphere.
Contact person: Prof. Zheng-Xiang Li, Earth Dynamics Research Group, Curtin University.
Xiaodian Jiang, Zheng-Xiang Li, Chaoyang Li, Wei Gong. A gravity study of the Longmenshan Fault Zone: New insights into the nature and evolution of the fault zone and extrusion‐style growth of the Tibetan Plateau since 40Ma. Tectonics, Volume 38, pages 176– 189 (2019). doi: 10.1029/2018TC005272